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NCERT Solutions Class 11 Biology Chapter 8 - Cell: The Unit of Life
NCERT Solutions for Class 11 Biology Chapter 8, Cell: The Unit of Life, explain the structure and functions of cells, which are the basic structural and functional units of living organisms. The chapter covers important topics such as Cell Theory, prokaryotic and eukaryotic cells, cell membrane, cell wall, cell organelles, and the nucleus. Students can understand how different parts of a cell are organised and how they perform specific functions.
NCERT Solutions for Class 11 Biology Chapter 8 by ALLEN provide clear explanations of important cell organelles such as mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, and nucleus. Students can also understand the Fluid Mosaic Model, the differences between prokaryotic and eukaryotic cells, and the structural differences between plant and animal cells. The solutions follow the concepts covered in NCERT Class 11 Biology Chapter 8 and help students prepare for questions based on cell structure and functions.
1.0Class 11 Biology Chapter 8 Cell: The Unit of Life: Key Concepts
Class 11 Biology Chapter 8, Cell: The Unit of Life, explains the structure and functions of cells and the different cell organelles found in prokaryotic and eukaryotic cells. The key concepts covered in this chapter include:
- Cell Theory: Understand cells as the basic structural and functional units of life and learn about the Cell Theory proposed by Schleiden and Schwann and later modified by Virchow.
- Prokaryotic Cells: Study the structure of bacterial cells, including the cell envelope, ribosomes, and inclusion bodies.
- Eukaryotic Cells: Understand the detailed internal organisation of eukaryotic cells and the functions of their different cell organelles.
- Cell Membrane: Learn about the structure of the cell membrane and the Fluid Mosaic Model proposed by Singer and Nicolson.
- Cell Wall: Study the structure and composition of cell walls in plants, fungi, and algae.
- Endomembrane System: Understand the structure and functions of the endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles and how they work together within the cell.
- Mitochondria and Plastids: Learn about the structure and functions of mitochondria and the different types of plastids, including chloroplasts, chromoplasts, and leucoplasts.
- Ribosomes and Cytoskeleton: Understand the role of ribosomes in protein synthesis and the structure and functions of cytoskeletal components such as microtubules and microfilaments.
- Cilia, Flagella, and Centrosomes: Study their structure and roles in cell movement and cell division.
- Nucleus: Understand the structure of the nuclear envelope, nucleoplasm, nucleolus, and chromatin, along with their functions.
- Microbodies: Learn about microbodies, which are small membrane-bound organelles found in cells.
2.0NCERT Solutions Class 11 Biology Chapter 8 -Cell: The Unit of Life: Detailed Solutions
- Which of the following is not correct? (a) Robert Brown discovered the cell. (b) Schleiden and Schwann formulated the cell theory. (c) Virchow explained that cells are formed from pre-existing cells. (d) A unicellular organism carries out its life activities within a single cell. Ans. (a) Robert Brown discovered the cell.
- New cells generate from (a) bacterial fermentation (b) regeneration of old cells (c) pre-existing cells (d) abiotic materials. Ans. (c) pre-existing cells
- Match the following.
Ans. a - (ii); b - (iii); c - (i).
- Which of the following is correct? (a) Cells of all living organisms have a nucleus. (b) Both animal and plant cells have a well defined cell wall. (c) In prokaryotes, there are no membrane bound organelles. (d) Cells are formed de novo from abiotic Ans. (c) In prokaryotes, there are no membrane bound organelles.
- What is a mesosome in a prokaryotic cell? Mention the functions that it performs. Ans. • Mesosome is a membranous structure in prokaryotic cell, which is formed by the extensions of the plasma membrane into the cell in form of vesicles, tubules and lamellae.
- Mesosomes are equal to mitochondria in eukaryotes, as they perform aerobic cellular respiration in prokaryotes.
- It helps in DNA replication and distribution of genetic material to daughter cells.
- Mesosomes also help in respiration, increase the surface area of the plasma membrane and enzymatic content.
- How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane? Ans. Neutral solutes move across the membrane by the process of simple diffusion along the concentration gradient i.e., from higher concentration to the lower concentration. Polar molecules cannot pass through the nonpolar lipid bilayer, they require carrier protein of the membrane to facilitate their transport across the membrane.
- Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both? Ans. Mitochondria and chloroplast are double membrane bound organelles. Mitochondria : Mitochondria are cylindrical or sausage shaped cell organelles and contains two membranes, outer and inner. The inner compartment is filled with a dense homogenous substance called the matrix containing single circular DNA, RNA, 70'S ribosomes, and the components required for the synthesis of proteins and outer membrane forms the continuous limiting boundary of the organelle. Inner membrane forms number of infoldings called the cristae which increases the surface area. Oxysomes are present on inner mitochondrial membrane. Mitochondria are semiautonomous organelles, i.e., have their own DNA and ribosomes.
Functions of Mitochondria:
- Mitochondria are the site of aerobic respiration.
- They produce cellular energy in the form of ATP, hence they are called power henses of the cell.
- Mitochondrial matrix contains enzymes for the synthesis of fatty acids.
- Synthesis of many amino acids takes place here Chloroplast : They are green coloured plastids which are disc shaped. The space limited by inner membrane of chloroplast is called as stroma. Stroma has organised flattened membranous sacs called the thylakoids. Thylakoids are arranged in stacks like the piles of coins called grana. Matrix of a chloroplast contains DNA, RNA, ribosomes and enzymes. Chloroplast is also a semiautonomous organelle.
Structure of a chloroplast
Functions of Chloroplast :
- Photosynthesis is performed by chloroplast.
- Chloroplast contain chlorophyll and carotenoid pigments.
- Maintains balance of concentration in the air.
- Keeps oxygen balance constant in atmosphere by liberating into the atmosphere, used during respiration and combustion.
- What are the characteristics of prokaryotic cells? Ans. Characteristics of prokaryotic cells are as follows:
- Prokaryotes have 70'S ribosomes.
- DNA is naked and lies coiled in cytoplasm. It is not covered by nuclear membrane and is termed as nucleoid or genomic DNA.
- In addition to the genomic DNA (the single chromosome/circular DNA) many bacteria have small circular DNA outside the genomic DNA called plasmid.
- Nuclear components, like, nuclear envelope, nucleolus, nucleoplasm are absent.
- Cell wall is present in bacteria and cyanobacteria, but absent in mycoplasma.
- Multiplies by asexual reproduction.
- Transcription and translation takes place in cytoplasm.
- Multicellular organisms have division of labour. Explain.
Ans. Division of labour is differentiation of certain components or parts to perform different functions for increased efficiency and higher survival.
- Multicellular organisms often possess millions of cells. Various cells are grouped together to form specific tissue, organ or organ system, with each specialised to perform particular function.
- Every cell of a multicellular organism cannot obtain food from outside. The organism requires a system for obtaining food, its digestion and distribution. Therefore, a digestive system and system of transport are also required.
- Certain cells of the body take over the function of reproduction.
- Others take part in repair and replacement
- For optimum functioning of cells also requires an internal favourable environment.
- Cell is the basic unit of life. Discuss in brief.
Ans. Cell is fundamental, structural and functional unit of life, as no living organism can have life without being cellular.
- An organism is either made of single cell (unicellular) or many cells (multicellular).
- In unicellular organism, single cell is capable of independent existence and perform all essential functions of life, while in multicellular organism, each group of cells is specialised for specific function.
- Life passes from one generation to the next in form of cells, and new cell always arise from division of pre-existing cells.
- Cells are totipotent, i.e., single cell has ability to form whole organism.
- The activities of an organism are sum total of activities of its cells, therefore, cell is the basic unit of life.
- What are nuclear pores? State their function.
Ans. The nuclear envelope is interrupted by minute nuclear pores, at a number of places, which are produced by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
- Both lysosomes and vacuoles are endomem-brane structures yet they differ in terms of their functions. Comment.
Ans. Organelles of endomembrane system such as lysosome and vacuoles function in close coordination with one another but are specialised to perform different functions. Lysosomes breakdown the ageing and dead cells, they help in digestion of food as they contain hydrolytic digestive enzymes. They are involved in cell division also. Vacuoles on other hand, help in excretion and osmoregulation in Amoeba (contractile vacuole).
- Describe the structure of the following with the help of labelled diagrams. (i) Nucleus (ii) Centrosome
Ans.
(i) Nucleus: Nucleus is double membrane bound principle cell organelle which contains all genetic information for controlling cellular metabolism and transmission of genetic information.
Nucleus is differentiated into following parts :
(a) Nuclear envelope : It is a double membrane bound envelope that surround the nucleus and separates the latter from the cytoplasm. (b) Nucleoplasm : The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis. (c) Chromatin : It has the ability to get stained with certain basic dyes. It is known to be the hereditary DNA protein fibrillar complex. The chromatin fibres are distributed throughout the nucleoplasm.
(ii) Centrosome: Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other. They are made up of nine evenly spaced peripheral fibrils of tubulin protein. Each of the peripheral fibril is a triplet. The adjacent triplets are also linked. The hub of centriole is connected with tubules of the peripheral triplets by radial spokes made of protein.
- What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes. Ans. A chromosome consists of two identical halves, the chromatids held together at one point called the centromere. The centromere is also called as primary constriction. On its side a disc shaped structure called kinetochore is present. Chromosomes are classified into four types according to position of centromere on the chromosome. (i) Metacentric chromosome : In this chromosome, centromere is in the middle and the two arms are almost equal in length. (ii) Submetacentric chromosome : The centromere is slightly away from middle point so one arm is slightly shorter than the other. (iii) Acrocentric chromosome : The centromere is near the end and one arm is extremely short and other arm is extremely long. (iv) Telocentric chromosome : Centromere is at the tip of chromosome. These chromosomes are not present in humans.
3.0NCERT Class 11 Biology – Chapter-Wise Solutions & Explanations
Explore chapter-wise NCERT Solutions for Class 11 Biology with easy answers to every textbook question. Learn important concepts, revise key topics, and improve your Biology preparation for exams.
4.0Key Features of NCERT Solutions for Class 11 Biology Chapter 8
- Structure and Function: Clear explanations connecting the structure of cell organelles with their functions, such as the role of cristae in mitochondria in providing a larger surface area for cellular respiration.
- Fluid Mosaic Model: Simple explanation of the lipid bilayer and membrane proteins to help students understand the structure and movement of substances across the cell membrane.
- Comparative Study: Clear comparisons of prokaryotic and eukaryotic cells and plant and animal cells to help students understand their structural differences.
- Semi-Autonomous Organelles: Explanation of why mitochondria and chloroplasts are considered semi-autonomous organelles, including their own DNA and 70S ribosomes.
- Diagram-Based Learning: Clear explanations of important cell structures, including the 9+2 arrangement in cilia and flagella and the 9+0 arrangement in centrioles.
- Complete NCERT Exercise Coverage: Answers to the NCERT exercise questions with clear explanations based on the concepts covered in Class 11 Biology Chapter 8.
- Prepared by Expert Faculty: Prepared by ALLEN subject experts with explanations aligned with the concepts given in NCERT Class 11 Biology Chapter 8.